Introduction Theory of Indicators Refference

1. THEORY OF INDICATORS
Prepared by
G. Nikitha, M.Pharmacy
Assistant Professor
Department of Pharmaceutical Chemistry
Sree Dattha Institute of Pharmacy
Hyderabad

2. CONTENTS
 Introduction
 Theory of Indicators
 Refference

3. INTRODUCTION
 An indicator is a substance which is used to determine the end point in a
titration.
 In acidbase titrations, organic substances (weak acids or weak bases) are
generally used as indicators.
 They change their colour within a certain pH range.

4. The colour change and the pH range of some common indicators are tabulated
below
Indicator pH range Colour in acid Colour in base
Phenolphthalein 8.3-10.5 Colourless Red
Litmus 5.5-7.4 Red Red
Methyl red 4.5-6.5 Red Yellow
Methyl Orange 3.2-4.5 Pink Yellow

5. THEORY OF ACID-BASE INDICATORS
Two theories have been proposed to explain the change of colour of acid-base
indicators with change in pH.
1) Ostwald's theory
2) Quinonoid theory

6. OSTWALD'S THEORY
According to this theory:
(a) The colour change is due to ionisation of the acid- base indicator. The unionised
form has different colour than the ionised form.
(b) The ionisation of the indicator is largely affected in acids and bases as it is
either a weak acid or a weak base.
In case, the indicator is a weak acid, its ionisation is very much low in acids due to
common H+ ions while it is fairly ionised in alkalies. Similarly if the indicator
is a weak base, its ionisation is large in acids and low in alkalies due to common
OH- ions.

7.  Ostwald's theory is based on Colour change
1. Phenolphthalein
2. Methyl orange
Weak acid Indicator Weak Base Indicator
Phenolphthalein(weak acid)

8. Weak acid Indicator Weak Base Indicator
Methyl orange

9. QUINONOID THEORY
 According to quinonoid theory, an acid-base indicators exist in two tautomeric
forms having different structures which are in equilibrium.
 One form is termed benzenoid form and the other quinonoid form.
 The two forms have different colors. The color change is due to the
interconversation of one tautomeric form into other. One form mainly exists in
acidic medium and the other in alkaline medium.
 Thus, during titration the medium changes from acidic to alkaline or vice-versa.
The change in pH converts one tautomeric form into other and thus, the colour
change occurs.

10. Quinonoid theory is based on structural change
1. Benzenoid form
2. Quinonoid Form

11. REFERENCE
 Pharmaceutical Chemistry -Inorganic Volume-1 by G. R. Chatwal.
 Essentials of Inorganic Chemistry by Katja A. Strohfeldt.
 Indian Pharmacopoeia.
 M.L Schroff, Inorganic Pharmaceutical Chemistry.
 P. GunduRao, Inorganic Pharmaceutical Chemistry, 3rd Edition
 A.I. Vogel, Text Book of Quantitative Inorganic analysis.
 Bentley and Driver's Textbook of Pharmaceutical Chemistry

12. THANK YOU